transformers-compat: A small compatibility shim exposing the new types from transformers 0.3 and 0.4 to older Haskell platforms.

This package includes backported versions of types that were added
to transformers in transformers 0.3 and 0.4 for users who need strict
transformers 0.2 or 0.3 compatibility to run on old versions of the
platform, but also need those types.

Those users should be able to just depend on transformers >= 0.2
and transformers-compat >= 0.3.

Note: missing methods are not supplied, but this at least permits the types to be used.